Diagnostic Challenge: An Extremely Rare Case of Intra-Articular Myopericytoma of the Knee

We report an extremely rare case of intra-articular myopericytoma of the knee. A 66-year-old man presented with a 10-year history of knee pain and a slowly enlarging soft, tender, and elastic mass. Magnetic resonance imaging revealed a well-enhanced intra-articular lesion; however, the findings were nonspecific and raised a broad differential diagnosis of vascular, perivascular, and malignant soft-tissue tumors. Incisional biopsy followed by marginal excision confirmed the diagnosis of myopericytoma. Although typically benign and indolent, myopericytomas can mimic both benign and malignant lesions, necessitating histopathological evaluation. At 5 years after surgery, the patient remained recurrence-free.